The insulation gas typically used in such switchgear is sulfur hexafluoride (SF6). In order to combat global warming due to greenhouse gas emission, the current trend is to mix the SF6 with some other gas such as nitrogen (N2) or such as carbon fluoride (CF4). That type of two-component mixture also improves the breaking performance of electrical switchgear at very low temperatures (−50° C.). It is possible that mixtures having more than two components might be used in the future.
The ratio of the SF6/N2 or SF6/CF4 mixture lies approximately in the range 50/50 to 80/20. To maintain a breaking capacity that is satisfactory in electrical switchgear that is insulated with a gas mixture of the N2/SF6 or CF4/SF6 type, it is essential for the proportion of N2 or of CF4 in the gas mixture to remain constant even in the event of leakage. Differential losses between the two components of the mixture can give rise to loss of performance in terms of breaking capacity.
There is also a need for manufacturers of gas-insulated electrical switchgear to specify accurately the ratio of the mixture after filling, in particular in order to satisfy the switchgear rating conditions.
To determine the mixture ratio or else the proportion of one component in a two-component gas mixture, it is known that chromatography or acoustic techniques can be used. However, those methods remain limited to use in the laboratory, and they are not applicable to on-site monitoring of the insulation gas in electrical switchgear. In addition, those methods are “intrusive” because they require the gas mixture to be tapped, which is not compatible with the operating conditions of gas-insulated electrical switchgear.